1. FIELD OF THE INVENTION
The present invention relates to an improved process for producing an aqueous solution of calcium nitrite by contacting a nitrogen oxides containing gas which contains NO and NO.sub.2 with an aqueous slurry of calcium hydroxide to absorb the gas.
2. DESCRIPTION OF THE PRIOR ART
Heretofore, calcium nitrite has been used as an anticorrosive agent and an additive for a cement etc. In industrial usages, calcium nitrite is preferably used in a form of an aqueous solution rather than a solid form, and especially in a form of an aqueous solution having about 30 to 40 wt.% of calcium nitrite.
The aqueous solution of calcium nitrite can be easily obtained by dissolving a solid calcium nitrite into water. The commercially available solid calcium nitrite is obtained by concentrating and drying an aqueous solution of calcium nitrite and accordingly, the method of dissolving the solid calcium nitrite is in remarkably low efficiency.
It has been proposed to produce an aqueous solution of calcium nitrite in Japanese Patent Publication No. 35596/1976. However, the known process requires many complicated steps such as a filtration, an aging, a concentration, a second filtration, and a second concentration, etc. to be low efficiency and to give large loss of the starting materials of a nitrogen oxides gas and an aqueous slurry of calcium hydroxide, disadvantageously.
The inventors have studied to attain a process for producing an aqueous solution of calcium nitrite having high purity and high concentration from a nitrogen oxides containing gas and an aqueous slurry of calcium hydroxide, and have found the fact that a formation of a by-product of calcium nitrate can be prevented by contacting a gas having less than about 3 vol.% of a nitrogen oxides concentration with an aqueous slurry having 3 to 10 wt.% of a calcium hydroxide content and containing calcium nitrite to absorb it whereby calcium nitrite is produced at high efficiency. The present invention has been attained by the finding.